Abstract:

A method includes inserting a virtual circuit connectivity verification
packet into aggregated traffic of m packets transmitted over multiple
pseudowires in a network path, replying to the transmitted m packets with
a virtual circuit connectivity verification packet with a packet loss
indication when at least one packet loss is detected in an m packet
group, and adjusting a rate of transmitting the aggregated m packets
responsive to the packet loss indication. In the preferred embodiment the
multiple pseudowires are one of constant bit rate and variable bit rate
and the adjusting of the rate includes rate adjustment of variable bit
rate pseudowires.

Claims:

1. A method comprising the steps of:inserting a virtual circuit
connectivity verification packet into aggregated traffic of m packets
transmitted over multiple pseudowires in a network path;replying to the
transmitted m packets with a virtual circuit connectivity verification
packet with a packet loss indication when at least one packet loss is
detected in an m packet group; andadjusting a rate of transmitting the
aggregated m packets responsive to the packet loss indication.

2. The method of claim 1, wherein the multiple pseudowires are one of
constant bit rate and variable bit rate.

5. The method of claim 4, wherein the rate adjustment of variable bit rate
pseudowires comprises adjustment in accordance with ×
##EQU00002## where Ti(n) is round trip time (RTT) of the variable
bit rate pseudowire VBR PW i in interval n; Ri(n) is rate of VBR PW
i in interval n; Di(n) is the number of successfully received
packets of VBR PW i in interval n; L is the virtual circuit connectivity
verification VCCV insertion interval.

6. The method of claim 5, wherein the step of adjusting a rate of
transmitting the aggregated m packets varies instantaneous changes in
round trip time.

8. A method comprising the steps of:inserting by a sending node over a
network path a virtual circuit connectivity verification packet into
aggregated traffic of a number of packets over multiple pseudowires being
one of a constant bit rate and a variable bit rate;replying to the
sending node with a virtual circuit connectivity verification packet with
a packet loss indication from a receiving node when at least one packet
loss is detected in the number of packets; andadjusting a rate of a rate
of the variable bit rate pseudowires responsive to the packet loss
indication received by the sending node.

10. The method of claim 9, wherein the rate adjustment of variable bit
rate pseudowires comprises adjustment in accordance with ×
##EQU00003## where Ti(n) is round trip time (RTT) of the variable
bit rate pseudowire VBR PW i in interval n; Ri(n) is rate of VBR PW
i in interval n; Di(n) is the number of successfully received
packets of VBR PW i in interval n; L is the virtual circuit connectivity
verification VCCV insertion interval.

11. The method of claim 10, wherein the step of adjusting the rate of the
number of packets varies with instantaneous changes in round trip time.

12. The method of claim 8, wherein the step of adjusting the rate
comprises rate adjustment of variable bit rate pseudowires until k
consecutive virtual circuit connectivity verifications without a packet
loss indication are received by the sending node.

Description:

[0001]This application claims the benefit of U.S. Provisional Application
No. 60/956,020, entitled "VCCV Insertion for packet Loss Control in
Pseudowire", filed on Aug. 15, 2007, the contents of which is
incorporated by reference herein.

BACKGROUND OF THE INVENTION

[0002]Pseudowire (PW) technology emulates the legacy services over a
packet switched network (PSN). The legacy services include Ethernet,
Frame Relay, PPP, HDLC, ATM, low-rate TDM, SONET/SDH, and Fiber Channel,
while PSN could be MPLS or IP (either IPv4 or IPv6). Legacy services have
been providing voice and data connectivity to businesses, end users, as
well as operators worldwide for years. T1 and E1 services have accounted
for a substantial proportion of carrier revenue, and will continue to do
so in the near future. The continued importance of the legacy services
requires a technology to facilitate their integration with PSN, and PW is
thus deemed as the evolutionary solution.

[0003]PW emulates the operation of traditional circuit connection by
carrying the legacy services through a PSN. As shown by the block diagram
10 in FIG. 1, pseudowire PW creates a point-to-point link, providing a
single TDM service which is perceived by its user as an unshared T1/E1
and T3/E3 circuit. In FIG. 1, the legacy service means a T1, E1, T3, or
E3 signal, while the PSN could be based on IP or MPLS network. Besides
TDM services illustrated in FIG. 1, PW is capable of supporting other
native services, such as Frame Relay, ATM, SONET/SDH, as well as Fiber
Channel. Because the PW emulation needs to satisfy the carried service
operation, cost-performance trade-off is necessary to balance between
circuit connection quality and packet switching capacity.

[0004]The IETF Pseudowire Emulation Edge to Edge (PWE3) working group was
set up in 2001, focusing on the architecture for service provider
edge-to-edge PW, and service-specific documents detailing the
encapsulation techniques. The most important PW standards include IETF
RFC 3985 ("Pseudo Wire Emulation Edge-to-Edge (PWE3) Architecture"), IETF
RFC 4447 ("Pseudowire Setup and Maintenance Using the Label Distribution
Protocol (LDP)"), IETF RFC 4448 ("Encapsulation Methods for Transport of
Ethernet over MPLS Networks"), and IETF RFC 4553 ("Structure-Agnostic
Time Division Multiplexing (TDM) over Packet"). Other standardization
forums, including the ITU, are also active in producing standards and
implementation agreements for PW.

[0005]Conventional networks support legacy services by employing specific
Operation, Administration, and Maintenance (OAM) mechanisms. When network
operators deploy the PW technology, seamless integration of these OAM
mechanisms must be taken into account. One critical issue that must be
addressed in PW is packet loss control. For example, in the conventional
circuit switched network, TDM services are provided over dedicated
channels with constant rates. Bit errors occur in the circuit switched
network, while packet losses are usually negligible. On the other side,
when packet switched network is employed for PW transmission, all PSNs
suffer packet losses. Packet losses in PW occur when the input PW traffic
requires more network resource than the PSN tunnel capacity. This is
especially the case when the incoming PW traffic includes TDM PW as well
as packet PW. The packet PW could carry non-congestion controlled
traffic, such as MPEG-2 streams, which are bursty in nature. When the
bursts of packet PW traffic overwhelm the PSN tunnel, packet losses are
inevitable, thus degrading the PW circuit emulation quality. Another
example is data synchronization. Native TDM data carry highly accurate
timing information for clock recovery. When emulating TDM over PW,
inevitable packet losses result in timing information losses, and
therefore, the inability to reproduce the TDM timing. To this end, packet
loss detection and control mechanisms for the PSN portion are pivotal to
the success of PW.

[0006]The Virtual Circuit Connectivity Verification (VCCV) mechanism was
recently proposed to facilitate OAM in PW. It defines a set of messages
which are inserted into PW data stream to enable management
functionalities, such as connectivity and verification. Each VCCV packet
contains the information of its sequence number as well as the current
value of the transmission counter for PW packets. When the PW receiver
receives a VCCV packet, it records the transmission counter contained in
the VCCV packet. Each PW receiver also has a local received counter,
which counts the received PW packets. The PW receiver compares the value
of the transmission counter with that of the received counter. Packet
losses are detected when the count of the transmitted packets is greater
than that of the received packets. VCCV provides the aforementioned
mechanism to detect packet losses, while the issues such as packet loss
control and compensation are open to and challenging the PW community.

[0007]Accordingly, there is need for a packet loss control mechanism in
pseudowire emulation by employing the VCCV messages.

SUMMARY OF THE INVENTION

[0008]In accordance with the invention, a method includes inserting a
virtual circuit connectivity verification packet into aggregated traffic
of m packets transmitted over multiple pseudowires in a network path,
replying to the transmitted m packets with a virtual circuit connectivity
verification packet with a packet loss indication when at least one
packet loss is detected in an m packet group, and adjusting a rate of
transmitting the aggregated m packets responsive to the packet loss
indication. In the preferred embodiment the multiple pseudowires are one
of constant bit rate and variable bit rate and the adjusting of the rate
includes rate adjustment of variable bit rate pseudowires.

[0009]In another aspect of the invention, a method for includes inserting
by a sending node over a network path a virtual circuit connectivity
verification packet into aggregated traffic of a number of packets over
multiple pseudowires being one of a constant bit rate and a variable bit
rate, replying to the sending node with a virtual circuit connectivity
verification packet with a packet loss indication from a receiving node
when at least one packet loss is detected in the number of packets; and
adjusting a rate of a rate of the variable bit rate pseudowires
responsive to the packet loss indication received by the sending node.
Adjusting the rate includes rate adjustment of the variable bit rate
pseudowires until consecutive virtual circuit connectivity verifications
without a packet loss indication occur.

BRIEF DESCRIPTION OF DRAWINGS

[0010]These and other advantages of the invention will be apparent to
those of ordinary skill in the art by reference to the following detailed
description and the accompanying drawings.

[0011]FIG. 1 is a diagram of an exemplary pseudowire emulation of legacy
services over a packet switched network (PSN).

[0012]FIG. 2 is a diagram illustrating an exemplary use of virtual circuit
connectivity verification (VCCV) messages at pseudowire PW provider edge
nodes for synchronization as well as packet loss control in accordance
with the invention.

[0014]The invention is directed to a packet loss control process to
emulate the delivery of native services over packet switched network
(PSN) using pseudowire (PW) technology. Virtual Circuit Connectivity
Verification (VCCV) messages are employed at the PW provider edge nodes
for synchronization as well as packet loss control. A VCCV insertion (VI)
process is used for adapting variable bit rate (VBR) PW rates to the
dynamics of PSN tunnel performance. Particularly, the rate adjustment
process employed at the provider edge nodes adapts PW data transmission.
This inventive technique classifies the PW traffic into two categories,
constant bit rate (CBR) and variable bit rate (VBR), and manages their
data delivery accordingly. It targets reduction of packet losses in PW by
tuning the VBR traffic delivery dynamically.

[0015]Packet Loss Control (PLC)

[0016]As shown by the block diagram 20 in FIG. 2, the PW provider edge
node classifies the incoming PWs into two categories, constant bit rate
pseudowire CBR PW, which carries data with strict requirement on
transmission rate and time synchronization, and variable bit rate
pseudowire VBR PW, which has relatively loose requirements on rate and
timing. A PSN tunnel is set up between two PW provider edge nodes,
containing all of the PWs between the two nodes. Sending node A inserts
VCCV packets into the PSN tunnel. As exemplified in FIG. 2, one VCCV
packet is inserted after three data packets. At receiving node B, packet
losses are detected by checking the VCCV packets as introduced in Section
2. In the reverse tunnel from B to A, VCCV packets are sent to verify the
forward transmission from A to B. When packet loss occurs, node B marks
the replied VCCV with a "packet loss" indication.

[0017]Assume multiple PWs are established within one PSN tunnel and the
network resource allocated to this tunnel is fixed. The packet loss
control PLC is implemented via the following three phases:

[0018]Phase 1: All PWs in the tunnel are put into two categories: CBR PWs
and VBR PWs. The sending node inserts one VCCV packet into the aggregated
tunnel traffic by every m packets.

[0019]Phase 2: After receiving m packets, the receiving node replies a
VCCV packet to the sending node. When one or more than one packet loss is
detected in an m packet group, a VCCV packet with "packet loss"
indication is replied.

[0020]Phase 3: When a VCCV packet is received by the sending node, the
flowchart 30 of FIG. 3 is employed for rate adjustment.

[0021]VCCV Insertion (VI)

[0022]As shown by the flow diagram 30 in FIG. 3, the VI process is
employed whenever packet loss 31 occurs in the PSN tunnel. The sending
edge node keeps tuning the rates of VBR PWs until k consecutive VCCV
without "packet loss" indication are received 32, 33, 34. Following
notations are adopted by the VI algorithm:

Ti(n): Round trip time (RTT) of VBR PW i in interval n;Ri(n):
Rate of VBR PW i in interval n;Di(n): Successfully received packets
of VBR PW i in interval n;L: VCCV insertion interval.

[0023]VI is employed at the sending edge node to throttle the VBR PW rate
32 as

× ##EQU00001##

When the PSN tunnel performance changes, PW circuit emulation falls into
one of three scenarios. First, in the reverse tunnel, when the VCCV
packets change from "no packet loss" indication to "packet loss"
indication, packet loss occurs on the forward tunnel. In this scenario,
RTT increases because of the overwhelmed traffic in the tunnel, and
Ti(n-1)<<Ti(n). As compared to Eq. (1), RTT Ti(n)
increases, in the meanwhile, Di(n) may reduce for packet losses in
VBR PW i. Therefore, the rate of VBR PW i is adjusted as
Ri(n+1)<Ri(n).

[0024]In the second scenario, the sending edge node keeps receiving VCCV
packets with "packet loss" indication 31, and VBR PW continues updating
its rate based on Eq. (1). VBR PW enters the tuning stage, where its rate
varies depending on the changes of instantaneous RTT and successively
received packets.

[0025]In the third scenario, packet loss has been eliminated by the rate
reduction of VBR PWs, the PSN tunnel performance becomes better with the
indication of a shorter RTT and no packet loss 34. In this scenario, we
have Ti(n-1)<<Ti(n). As compared to Eq. (1), RTT
Ti(n) decreases, and Di(n) may increase for packet loss
elimination in VBR PW i. The rate of VBR PW i is thus adjusted as
Ri(n+1)>Ri(n).

[0026]We have proposed a mechanism to address the issue of packet loss
control in PW. It employs VCCV packets for PSN tunnel performance
monitoring. The proposed VI algorithm throttles VBR PW rates when packet
losses occur in the PW circuit emulation.

[0027]The present invention has been shown and described in what are
considered to be the most practical and preferred embodiments. It is
anticipated, however, that departures may be made therefrom and that
obvious modifications will be implemented by those skilled in the art. It
will be appreciated that those skilled in the art will be able to devise
numerous arrangements and variations which, not explicitly shown or
described herein, embody the principles of the invention and are within
their spirit and scope.